Close binaries in near and shallow contact stages

Abstract

After a detailed investigation of new observations obtained by small telescopes, we found a group of near and shallow contact binaries with decreasing period including BS Vul, MR Com, EP Cep, ES Cep and V369 Cep. BS Vul and V369 Cep are primary-filling near-contact binaries, while MR Com, EP Cep and ES Cep are shallow contact binaries. With their decrease in period, BS Vul and V369 Cep will evolve to the shallow-contact systems with higher mass ratios, just like ES Cep. The period decease of shallow contact binaries could be caused by the combined effect of the thermal relaxation oscillation (TRO) and variable angular momentum loss (AML). This produces contact binaries in oscillating circles that evolve towards the lower mass ratio direction. That means systems resemble ES Cep will evolve to those systems with lower mass ratio, such as MR Com and EP Cep.